1. Field of the Invention
The invention relates to a superconducting magnet system with a superconducting magnet coil disposed in a cryostat with a cold shield, the magnet being formed of a material having a high critical temperature such as niobium/tin filaments embedded in a bronze matrix or niobium carbonitride films applied to carrier fibers, and the magnet coil being cooled to an operating temperature of about 10 to 13 K. by a two-stage cryogenerator.
2. Description of the Related Art
Such a magnet system has become known from the publication "IEEE Transactions on Magnetics", Vol. MAG-19, No. 3, May '83, pages 880 to 883. A superconducting coil made with Nb.sub.3 Sn conductors is described therein. The coil is operated in the temperature range between 12 and 14 K. It is cooled through heat conduction by a two-stage refigenerator operating according to the Gifford-McMahon process. Therefore, no auxiliary coolants are required. The lowest attainable temperature of such a cryogenerator is about 10 K. At this temperature, however, the apparatus is no longer able to furnish a cooling output worth mentioning, so that a higher operating temperature such as near 13 K. must be selected. At this temperature, a superconductor having an Nb.sub.3 Sn basis carries an adequately high current to produce coils for magnetic fields up to 3 or 4T. Superconductors having an NbCN basis also meet such a condition. The coil itself is thermally coupled to the second stage of the cryogenerator, whereas a cold shield for reducing thermal radiation ia attached to the first stage. Since no coolant (e.g. helium) is used, the current leads cannot be cooled by exhaust gas as usual. They are isothermally coupled to both stages of the cryogenerator so that cooling takes place through thermal conduction. In order to keep the heat losses as small as possible, the current in the leads must be kept to a minimum.
In the known prior art device, the leads consume a substantial part of the available cooling capacity and yet limit the maximally possible current density in the magnet coil. The reason for this is that no arbitrarily thin superconducting wires can be used for the magent coil. In the Nb.sub.3 Sn technique, for example, no economic method is known for producing superconductors with stabilizing copper and a diameter smaller than 0.8 mm. This is because, when drawing down to smaller diameters, there is the danger of damaging the required tantalum diffusion barrier between the superconductor and the stabilizing copper. However, the stabilizing copper is needed for the safe operation of the magnet coil. This leads to a situation wherein the superconducting wires of the magnet coil are able to carry a relatively large current which may be as high as several hundred amperes. However, such a large current would cause such high heat losses in the leads that the cooling capacity of the cryogenerator would be insufficient.
In order to be able to produce a magnetic field of 3 to 4T despite the limitation of the operating current of the current leads, a magnet coil having more windings than would be necessary based on the current carrying capacity of the superconducting wires could be used. This leads to an increased coil weight. However, the coil weight is limited by the cryogenerator selection because the cryogenerator's cold head must be in a position to support the coil weight and because, in addition, the thermally coupled coil mass should be coolable within a reasonable period of time, e.g. 1 to 3 days. This results in a weight limitation for the coil of 50 to 60 kg. An optimal coil construction is not possible for these reasons.
According to the state of the art, the cryogenerator cold head is disposed in the center of the coil so that this space is no longer available as a usable volume. However, many applications require the coil interior as a usable volume.